1. Field of the Invention
The present invention relates to ink jet printers in general, and in particular, to aspirated ink jet printers wherein there is collinear flow between a stream of air and the ink droplets emanating from the printer head.
2. Prior Art
The use of ink jet printers for printing data and other information on a strip of recording media is well known in the prior art. One type of conventional ink jet printer incorporates a plurality of electrical components and fluidic components. The components coact to enable the printing function. The fluidic components include a print head having a chamber for storing a printing fluid or ink and a nozzle plate with one or more ink nozzles interconnected to the chamber. A gutter assembly is positioned downstream from the nozzle plate in the flight path of ink droplets. The gutter assembly catches ink droplets which are not needed for printing on the recording medium.
In order to create the ink droplets, a drop generator is associated with the print head. The drop generator vibrates the head at a frequency which forces thread-like streams of ink, which are initially ejected from the nozzles, to be broken up into a series of ink droplets at a point (called the break-off point) within the vicinity of the nozzle plate. A charge electrode is positioned along the flight path of the ink droplets. Preferably, the charge electrode is positioned at the break-off point of the ink drolets. The function of the charge electrode is to selectively charge the ink droplets as said droplets pass said electrodes. A pair of deflection plates is positioned downstream from the charge electrodes. The function of the deflection plates is to deflect a charged ink droplet either into the gutter or onto the recording media.
Another type of conventional ink jet printer incorporates a plurality of magnetic components and fluidic components. The fluidic components are substantially equivalent to the fluidic components previously described. However, the electrical components are replaced with magnetic components for influencing the direction of the streams. This type of ink jet printer is well known in the prior art and, therefore, the details will not be described.
One of the problems associated with ink jet printers of the aforementioned types is that of ink droplet misregistration at the recording surface. The ink droplet misregistration arises from interaction between the droplets as said droplets are propelled along a flight path towards the recording surface. The causes for droplets interaction are usually twofold: namely, the aerodynamic drag on the respective droplets and the electrical interaction between the electrical charges which are placed on the ink droplets.
The aerodynamic interaction and the electrical interaction are closely related. In fact, the aerodynamic interaction and the electrical interaction are complementary and are usually never observed independently. As ink droplets are generated at the nozzle plate, the charge electrode deposits a certain quantum of electrical charge on the droplets. Depending on the polarity of the charge, the droplets either repel or attract one another. The electrical forces which attract and/or repel the ink droplets tend to affect the relative spacing between the droplets. As such, some droplets arrive at the recording media early while others arrive late. In some situations, the droplets arrive at the recording media in groups rather than individual drops. The net result is that the copy quality is relatively poor due to droplet misplacement on the media.
The aerodynamic interaction also tends to affect the relative spacing between droplets. Spacing is affected because the aerodynamic interaction either increases or decreases the velocity of the droplets. As a result, some ink droplets are reaching the media early while others are reaching the media late. The overall effect is that the presence of the aerodynamic interaction also called the aerodynamic drag, aggravates or magnifies the effect of the charge interaction.
The aerodynamic interaction, sometimes called the aerodynamic drag, also creates a nonuniform velocity in the streams emanating from a multinozzle head. Consequently, the velocity variation from stream to stream results in inaccurate placement of the ink droplet and poor print quality.
In order to effectively solve droplet registration problems, both the charge interaction and the aerodynamic interaction have to be addressed. The prior art uses the so-called guard drop method to solve the charge interaction problem. In this method nonadjcent droplets are charged. Stated another way, charged droplets are separated by a predetermined number of noncharged droplets.
In addressing the aerodynamic interaction problem, the prior art utilizes a gas stream, such as air, to compensate for the aerodynamic drag on the ink droplets. U.S. Pat. No. 3,596,275 is an example of the prior art method. In that patent a stream of air is introduced into the droplet flight path. The air flows collinearly, with the stream of ink droplets and reduces the aerodynamic effect. In order to maintain laminar air flow, beginning at the point where the droplets are interjected into the air stream or vice versa, the nozzle is mounted in the center of the air stream. The charging electrode is fabricated in the shape of a hollow streamline strut. The strut is fitted with an opening through which ink droplets are ejected. The strut surrounds the nozzle with its opening and streamline contour position in the direction of air flow.
U.S. Pat. No. 4,097,872 is another prior art example of an aspirator where a fluid such as air is used to correct for aerodynamic interaction or aerodynamic drag. The aspirator includes a housing having a tunnel therein. The tunnel is spaced from an ink jet nozzle which emits an ink stream which passes through the tunnel. The tunnel is characterized by a circular geometry with a settling chamber section and a flow section. Air turbulence is removed at the settling chamber. Although the use of air into the ink droplets' flight path to correct for aerodynamic drag on the droplets is a step in the right direction, the prior art ink jet printers occasionally reproduce poor quality prints. The cause for the poor quality prints stems from the inaccurate placement of ink droplets on the reproducing media. The inaccurate drop placement is due to a nonuniform velocity profile between the streams of the printer.
U.S. Pat. No. 4,045,770 describes an apparatus for adjusting the velocity between the droplets of a single nozzle magnetic ink jet printer system. In the system, a coarse control loop servo and a fine control loop servo makes coarse and fine incremental adjustments, respectively, to a pump which supplies ink under pressure to the single nozzle. The direction of a velocity error signal associated with the drops are measured by a pair of drop sensors positioned relative to the droplets' flight path. The drop sensors are separated by a spacing of one drop wave length apart at a fixed distance from the drop generation point. The error signal is used to activate control circuits associated with the coarse and fine control loop servos.
U.S. Pat. No. 3,787,882 describes another sero system for controlling the velocity of ink jet streams. In the patent, the temperature and/or pressure of the ink is sensed at the pump and appropriate adjustments are made to the pump driving circuit to increase or decrease pump pressure and thereby increase or decrease velocity of the stream. The patent further contemplates the sensing of stream velocity and generating an error signal which is used to activate the pump driving circuit.